Conventional single-stage seals, providing a single barrier against fluid leakage, are generally unsuitable for operation over wide ranging pressures. High pressure seals composed of single resilient rings tend to extrude into the clearance between piston and cylinder. Fluid under pressure can seep between the seal and the members which it engages. Extrusion can be cured by using relatively hard-sealing, non-resilient rings. However, non-resilient rings must be installed tightly for effectiveness at high pressures. Consequently, they are unsuited for use at low pressure, causing undue friction and wear.
The solution to the problem is a two-stage assembly comprising a sealing ring of resilient deformable material effective at low pressure (100-300 psi) and a mating back-up ring responsive at high pressure (500 psi or greater), which is composed of a slightly resilient material such as virgin or loaded TEFLON. The sealing ring engages the opposing relatively-reciprocating member forming a seal at low pressure. At higher pressures, the more resilient sealing ring will pull away from the relatively-reciprocating member and radially urge the back-up ring into tight-sealing contact with that member. The result is a sealing assembly effective at low and high pressures, without causing excessive friction and wear at low pressures.